Your logical fallacy here is that you have no evidence whatsoever that the people complaining are not voting for a third party every time. Perhaps they are, and in fact, it is as ineffectual as they claim.
even ignoring the issues i raised in my original post, another huge problem is the success of propaganda and the polarization of the political landscape. even if the system worked perfectly, a large portion of the population would continue to vote against their self-interest due to propaganda and social conditioning. and the polarization makes it so you can't get people to entertain different views, as they feel that they're getting attacked and "losing the fight" if they change their stance.